Photovoltaic cell

ABSTRACT

A photovoltaic cell is described, having a photoactive layer ( 4 ) made of two molecular components, namely an electron donor and an electron acceptor, particularly a conjugated polymer component and a fullerene component, and having two metallic electrodes ( 2, 6 ) provided on both sides of the photoactive layer ( 4 ). In order to provide advantageous construction conditions, it is suggested that an intermediate layer ( 5 ) made of a conjugated polymer, which has doping corresponding to the electrode potential and, in regard to the electron energy, has a band gap between the valence band and the conduction band of at least 1.8 eV, be provided between the photoactive layer ( 4 ) and at least one electrode ( 2,6 ).

[0001] The present invention relates to a photovoltaic cell having a photoactive layer made of two components, namely an electron donor and an electron acceptor, particularly a conjugated polymer component and a fullerene component, and having two metallic electrodes provided on both sides of the photoactive layer.

[0002] Plastics having extensive π-electron systems, in which single and double bonds follow one another alternately in sequence, are referred to as conjugated plastics. These conjugated plastics have energy bands which are comparable with semiconductors in regard to electron energy, so that they may also be transferred from the non-conductive state into the metallically conductive state through doping. Examples of such conjugated plastics are polyphenylenes, polyvinylphenylenes (PPV), polythiophenes, or polyanilines. The efficiency of energy conversion of photovoltaic polymer cells made of a conjugated polymer is, however, typically between 10⁻³ and 10⁻²%. To improve this efficiency, heterogeneous layers made of two conjugated polymer components have already been suggested (U.S. Pat. No. 5,670,791 A), one polymer component being used as an electron donor and the other polymer component as an electron acceptor. By using fullerenes, particularly buckminsterfullerenes C₆₀, as electron acceptors (U.S. Pat. No. 5,454,880 A), the charge carrier recombination otherwise typical in the photoactive layer may be largely avoided, which leads to a significant increase in efficiency. For reaching a good efficiency, good charge separation is a necessary, but not sufficient condition, because it must also be ensured that the separated charges also reach the corresponding electrodes of the photovoltaic cell. In typical photovoltaic cells of this type, a hole-collecting electrode made of indium/tin oxide (ITO) and an electrode-collecting electron made of aluminum have proven themselves.

[0003] The present invention is therefore based on the object of designing a photovoltaic cell of the type initially described in such a way that the charge transport between the photoactive layer and electrodes may be increased to elevate the short-circuit current.

[0004] The present invention achieves the object described in that an intermediate layer made of a conjugated polymer, which has a doping corresponding to the electrode potential and, in regard to the electron energy, has a band gap between the valence band and the conduction band of at least 1.8 eV, is provided between the photoactive layer and at least one electrode.

[0005] Since the conjugated polymer of the intermediate layer is doped according to the electrode potential, which means an oxidative doping in the region of the hole-collecting electrode and a reductive doping in the region of the electron-collecting electrode, the conjugated polymer ensures a hole excess in the region of the hole-collecting electrode, but an electron excess in the region of the electron-collecting electrode, so that, in the region of the oxidatively doped polymer, the hole conduction is reinforced, and the electron conduction is reinforced in the region of a reductively doped polymer. Since, however, the conjugated polymer of the respective intermediate layer has, in regard to the electron energy bands, a comparatively large band gap of at least 1.8 eV between the valence band and the conduction band, a correspondingly high activation energy results for the intrinsic conduction, which leads, in the case of an oxidatively doped polymer layer, to the electron conduction from the photoactive layer to the hole-collecting electrode being obstructed and, in the case of the reductively doped intermediate layer, to the hole conduction from the photoactive layer to the electron-collecting electrode being obstructed. With the aid of these separate intermediate layers, a valve effect may therefore be achieved, which supports the conduction of the charge carriers of the respective adjoining electrode which are to be collected from the photoactive layer to the electrode, but obstructs diffusion of opposing charges in the same direction. As a result of these separate layers, the charge conduction to the electrodes may correspondingly be improved, which directly results in an increase of the short-circuit current. It probably does not have to be emphasized that, depending on the application, photovoltaic cells may be used which have such an intermediate layer between the hole-collecting electrode and the photoactive layer, between the electron-collecting electrode and the photoactive layer, or in the region of both electrodes.

[0006] Although different conjugated polymers may be accordingly oxidatively or reductively doped to implement the intermediate layers, particularly advantageous ratios result if the intermediate layer is made of a doped polythiophene derivate. Both high polymers and oligomers are to be understood from the concept of polymer.

[0007] An example of the object of the present invention is illustrated in the drawing.

[0008]FIG. 1 shows a detail of a photovoltaic cell according to the present invention in a schematic section, and

[0009]FIG. 2 shows the current-voltage characteristics of a typical photovoltaic cell and a photovoltaic cell according to the present invention.

[0010] As shown in FIG. 1, the photovoltaic cell comprises a transparent glass carrier 1, onto which an electrode layer 2 made of indium/tin oxide (ITO) is applied. This electrode layer 2 generally has a comparatively rough surface structure, so that it is covered with a smoothing layer 3 made of a polymer, typically PEDOT, which is made electrically conductive through doping. In contrast to typical photovoltaic cells of this type, according to the present invention, photoactive layer 4 is not applied directly onto the smoothing layer, but rather onto an intermediate layer 5, which is made of conjugated polymer, preferably from a poly(3-alkylthiophene), which is oxidatively doped using nitrosonium tetrafluoroborate after being applied onto smoothing layer 3, in order to obtain a corresponding hole excess.

[0011] Photoactive layer 4, which is applied onto intermediate layer 5 in the form of a solution, is made of a conjugated polymer, preferably a polythiophene derivate, as an electron donor, and a fullerene, particularly functionalized fullerene PCBM, as an electron acceptor. The concept of polymer is to be understood to mean both high polymers and oligomers. Electron-collecting electrode 6 is made of aluminum, which, in the case of the exemplary embodiment illustrated, is vapor deposited onto photoactive layer 4 without interposing a further intermediate layer, which would, however, be completely possible. In this case, the conjugated polymer of the intermediate layer would be reductively doped corresponding to the negative potential of electron-collecting electrode 6, in order to ensure a corresponding electron excess.

[0012] Due to the intermediate layer 5, which has a band gap of at least 1.8 eV between the valence band and the conduction band, the entrance of electrons from photoactive layer 4 into intermediate layer 5 is made more difficult due to this comparatively broad band gap, without the hole conduction between photoactive layer 4 and hole-collecting electrode 2 being impaired. The conduction band of the conjugated polymer of intermediate layer 5 is, for example, in contrast to PEDOT layer 3, at a significantly higher energy level than the energy band of the electron acceptor of photoactive layer 4. This leads to a unipolar charge transfer from photoactive layer 4 to hole-collecting electrode 2, which is observable in a corresponding increase of the short-circuit current, as may be seen in FIG. 2. In FIG. 2, current density I is graphed over voltage U at an excitation energy of 80 mW/cm² under simulated AM 1.5 conditions for a photovoltaic cell according to the present invention in comparison to a cell constructed correspondingly, with the exception of intermediate layer 5. It is shown that characteristic 7, assigned to the photovoltaic cell according to the present invention, results in a short-circuit current, measured at voltage U=O V, which is approximately twice as large as the short-circuit current of the comparison cell shown in characteristic 8.

[0013] It probably does not have to be noted in more detail that, if intermediate layer 5 is positioned between photoactive layer 4 and electron-collecting electrode 6, the distance of the valence bands, and not of the conduction bands, is decisive.

[0014] Since the effect of electrically insulating transition layer 6 is not restricted to conjugated polymers as electron donors and fullerenes as electron acceptors, the effect according to the present invention may also be observed in all photovoltaic cells having a molecular two-component layer made of an electron donor and an electron acceptor. 

What is claimed is:
 1. A photovoltaic cell having a photoactive layer (4) made of two molecular components, namely an electron donor and an electron acceptor, particularly a conjugated polymer component and a fullerene component, and having two metallic electrodes (2, 6) provided on both sides of the photoactive layer (4), characterized in that an intermediate layer (5) made of a conjugated polymer, which has a doping corresponding to the electrode potential and, in regard to the electron energy, has a band gap between the valence band and the conduction band of at least 1.8 eV, is provided between the photoactive layer (4) and at least one electrode (2,6).
 2. The photovoltaic cell according to claim 1, characterized in that the intermediate layer (5) is made of a doped polythiophene derivate. 